The present invention generally relates to an optical low pass filter including a phase grating for obtaining useful picture image characteristics by provision preferable low pass effects for each of wavelength region about the spatial frequency components beyond the Nyguist limit which are required to be removed as the picture images are discretely sampled in nature in an optical system of a single tube color television camera having a color separating filter, a color video camera using solid state imaging elements, and so on. The present invention relates to an optical low pass filter which is easier to fabricate.
Generally, in a video camera and so on using solid state imaging elements, the spatial frequency components of the Nyquist limit or more should be removed before images are inputted into the solid imaging elements, because the images of the photographed objects image-formed on the imaging elements include the components beyond the Nyquist limit of the spatial sampling of the solid imaging elements. This is because the picture image signal components beyond the Nyquist limit display the construction not provided originally with respect to the output picture images or become Moire stripes, spurious colors and so on. Therefore, conventionally the higher spatial frequency components in the photographed object to be inputted to the solid state imaging elements are restricted by disposition of the optical low pass filters in one portion of the imaginating system. In recent years, optical low pass filters including phase gratings have been adapted to be used for this purpose.
In recent years, the picture images of a higher resolution have been obtained by the effective use of a fact that the spatial frequency of the green component of the primary color filter used in a single plate type of imaging system is higher than the spatial frequency components of red and blue. As disclosed in, for example, Japanese Laid-Open Patent Publication No. 2-113302, a phase grating optical low pass filter is composed of two layers, with a disadvantage in that it becomes thicker in construction. In the present invention, the construction can be made thinner with the thickness being restricted only to the phase difference generating portion. In the phase grating optical low pass filter in the present invention, an easier coping operation can be effected even with respect to optical line pass adjustment or mechanical adjustment.